<?php

$data=array(
	array('text'=>'str1', 'year'=>'2010', 'author'=>10),
	array('text'=>'str2', 'year'=>'2011', 'author'=>10),
	array('text'=>'str3', 'year'=>'2009', 'author'=>20),
	array('text'=>'str4', 'year'=>'2010', 'author'=>30),
	array('text'=>'str5', 'year'=>'2010', 'author'=>20),
	array('text'=>'str6', 'year'=>'2011', 'author'=>10),
	array('text'=>'str7', 'year'=>'2011', 'author'=>20),
	array('text'=>'str8', 'year'=>'2009', 'author'=>20),
);

print("<p>ДО</p>");

print("<pre>");
print_r($data);
print("</pre>");
/*
$data_author=array();
foreach($data as $key=>$arr){
	$data_author[$key]=$arr['author'];
}
*/ 
$data_year=array();
foreach($data as $key=>$arr){
	$data_year[$key]=$arr['year'];
}
 
array_multisort($data_year, SORT_NUMERIC, $data);

print("<p>ПОСЛЕ</p>");

print("<pre>");
print_r($data);
print("</pre>");

?>